Drilling fluids are widely used for the drilling of oil and gas wells. These drilling fluids provide suppression of reservoir pressure, lubrication of the drill pipe, effect cooling of the bottom hole drill assemblies and removal of the cuttings from down-hole. Down-hole assemblies may contain individual components such as bits, stabilizers, measurement while drilling tools, etc. Often down-hole assemblies contain electronic instruments that contain microprocessors that are used to collect and/or transmit data collected by various sensor arrays to the surface for analysis to determine conditions down-hole. Drilling fluids generally contain a variety of elements, both desirable and undesirable, such as mud, chemicals, drill cuttings, metal shavings, etc. The particle size of these various elements varies from a few microns to several inches. Additionally, rig crews often inadvertently drop tools, gloves, rags, and other foreign or unwanted materials into the drilling fluid tanks, which may be pumped, unnoticed, into the well bore. In addition, down-hole broken pump and valve parts, such as rubber gaskets and metal fragments, are often circulated through the tubular string from the mud reclamation tanks through the drill string. The unwanted and/or undesirable solid materials, referred to as debris, can be extremely harmful to down-hole tools, especially those containing instruments and the like. Therefore, it is desirable to filter or otherwise collect as much of such debris as possible from the drilling fluid at the drill floor. It has been the accepted practice to install filter screens at critical locations, such as at the entrance to the mud pump, but such filter screens have often proven inadequate and provide no protection from debris that passes though the tubing from the mud reclamation tanks. Although mud screens have been inserted into the mud tanks, they have the disadvantage of being cumbersome to install and difficult to remove or to clean if necessary. In spite of the operator's best efforts, some debris still gets past the surface filtration system.
A great many methods of filtering well bore fluids have been used with varying degrees of success. One such method includes placing various types of screens in the tubular joint being added to the string prior to coupling the joint to the Kelly while the tubular members are being run into the well bore. Such devices include a cylindrical or conical screen located within the Kelly joint temporarily and removed before the next joint is applied to the string. However, in some cases, the screen is inadvertently left in the joint and allowed to advance down-hole with the joint. Some of these screens have retrieval capability while down-hole without uncoupling the joint.
Since it is quite beneficial to screen the drilling fluid being pumped down-hole through the drill string at the drill floor in a manner that is supposed to eliminate any foreign debris from becoming entrained in the fluids that may plug or damage any of the down-hole tools or sensitive instruments located therein during drilling operations, one would assume that very little debris would accumulate within the down-hole strainer/collectors. However, when the strainer is allowed to remain in the joint down-hole, flow though the drill string is reduced, thus increasing the pressure requirement. If the down-hole screen becomes even partially blocked, the flow of fluids is even further reduced, thereby, reducing the necessary fluids to the drill bit and thus increasing wear. In some cases a second strainer is placed at the Kelly joint to help prevent debris from reaching the strainer left in the joint down-hole further reducing fluid flow.
In many cases, removal of the down-hole screen can only be accomplished by tripping the pipe out of the hole which, of course, becomes impossible in the event that the pipe string is stuck. If left in place, a down-hole screen will prevent a blockage to any tools, such as survey instruments, string shots, etc., that may be run any time during the drilling operation. Of course, fluid pressure through the drill string will increase due to resistance and the screen may eventually become plugged and severely limit the flow of fluid unless it is removed and cleaned occasionally. Down-hole type screens heretofore provided, although capable of removal, run an extreme risk that the debris collected by the screen will escape from the screen during the removal process and then plug the down-hole devices meant to be protected in the first place. Therefore, there is a need for a tool joint filtering device, which efficiently filters the drilling fluid while still allowing maximum flow-through capabilities. It is far better to retrieve a tool that has become inoperative due to a plugged filter than due to extreme damage by such debris.
In addition, since the down-hole drilling fluids are quite abrasive and are pumped at high pressures of approximately 5000 PSI, any reductions in filter slot size increases velocity. Therefore, filters and strainer disks having such slots suffer significant wear and breakage when subjected to such high pressure. It becomes obvious that in cases where high velocity is not required, slot size should be maximized and that more attention should be paid to the type of metal, surface preparation, and cladding to reduce abrasion wear.